He said the Fraser of Allander Institute predicted Brexit would cost the economy £11 billion a year by 2030 and lead to the loss of 80,000 jobs.

Mr Brown said Scotland should have a separate immigration policy and stressed freedom of movement was crucial to the economy.

Speaking at the same event, Scottish Conservative finance spokesman Murdo Fraser said predictions the UK economy would struggle following the Brexit vote had not come to pass, adding Scotland is the only area in the UK where the economy is struggling.

“I think going on and on and on about a second independence referendum is the most damaging thing for economic recovery in Scotland,” he added.

Meanwhile, cconomists are expecting a marked dent in Britain’s gross domestic product (GDP) when official figures are released tomorrow, with growth slowing to 0.4 per cent from 0.7 per cent in the fourth quarter, as consumers tighten their belts in the face of rising living costs.

The bleak outlook has been compounded by dismal retail sales, which recorded their biggest fall for seven years in the three months to March.

Consumers are feeling the pinch, with inflation sitting at its joint highest level for more than three years at 2.3% last month.

While falling flight and fuel prices kept a lid on the overall cost of living, price tags on food and clothing kept climbing in March.

Howard Archer, chief UK and European economist for IHS Markit, said the predicted first-quarter slowdown was “primarily the consequence” of consumers reining in their spending.

“Additionally, we expect businesses to become more cautious over investment, as the economy shows mounting signs of slowing and uncertainties over the outlook are magnified by Brexit negotiations coming increasingly to the forefront now that the Government has triggered Article 50,” he said.